In inkjet printing, ink droplets are released from an array of nozzles in a print head onto a printing medium, such as paper. The ink bonds to a surface of the printing medium and forms graphics, text, or other images. The ink droplets are released with precision to ensure that the image is accurately formed. Generally, the medium is conveyed under the print head while the droplets are selectively released. The medium's conveyance speed is factored into the droplet release timing.
Some inkjet printers include print heads that slide laterally across a swath, or width, of the printing medium during a print job. In such printers, the medium's conveyance is halted momentarily as the print head travels and releases the predetermined droplets along the swath of the medium. Other inkjet printers include print heads that remain stationary throughout a printing job. In these printers, an array of nozzles generally spans the entire swath of the printing medium.
Print heads typically include a number of ink chambers, also known as firing chambers. Each ink chamber is in fluid communication with one of the nozzles in the array and provides the ink to be deposited by that respective print head nozzle. Prior to a droplet release, the ink in the ink chamber is restrained from exiting the nozzle due to capillary forces and/or back-pressure acting on the ink within the nozzle passage. The meniscus, which is a surface of the ink that separates the liquid ink in the chamber from the atmosphere located below the nozzle, is held in place due to a balance of the internal pressure of the chamber, gravity, and the capillary force. The size of the nozzle passage is a contributing factor to the strength of the capillary forces. The internal pressure within the ink chamber is generally insufficient to exceed the strength of the capillary force, and thus, the ink is prevented from exiting the ink chamber through the nozzle passage without actively increasing the pressure within the chamber.
During a droplet release, ink within the ink chamber is forced out of the nozzle by actively increasing the pressure within the chamber. Some print heads use a resistive heater positioned within the chamber to evaporate a small amount of at least one component of the liquid ink. In many cases, a major component of the liquid ink is water, and the resistive heater evaporates the water. The evaporated ink component or components expand to form a gaseous drive bubble within the ink chamber. This expansion exceeds the capillary force enough to expel a single droplet out of the nozzle. Generally, after the release of single droplet, the pressure in the ink chamber drops below the strength of the capillary force and the remainder of the ink is retained within the chamber. Meanwhile, the drive bubble collapses and ink from a reservoir flows into the ink chamber replenishing the lost ink volume from the droplet release. This process is repeated each time the print head is instructed to fire.